From Riemann.
Washing Instructions
Many sun protection products can stain fabrics, and this also applies to P20. However, we have found that by following the washing instructions outlined below, these stains can be removed.
Treat the stains with a commercial stain remover (we recommend Vanish Oxi Action) and wash the fabric with an ordinary detergent. Follow the instructions closely for both products as well as the washing instructions for the fabric itself. Repeat the treatment if necessary. Avoid any use of chlorine as this may worsen the stains.
To achieve the best results, the fabric should be treated as soon as possible after staining occurs.